Claims:We Claim:

1. A hot rolled steel suitable for deep drawing applications in structural components comprising of a steel composition comprising of carbon between 0.0005 and 0.01%, Si between 0.95 and 2.5% and minor alloying with Al between 0.05 and 0.3% and rest Fe and the steel has single phase ferritic microstructure with yield strength range of 250 to 350 MPa, ultimate tensile strength (UTS) in the range of 380 to 520 MPa and elongation in the range of 23 to 30%.

2. The hot rolled steel suitable for deep drawing applications in structural components as claimed in claim 1, wherein the strength and specific strength of the steel increases with silicon content.

3. The hot rolled steel suitable for deep drawing applications in structural components as claimed in anyone of claims 1 to 2having strain hardening exponent (n) in the range of 0.18 to 0.22 and YS/UTS ratio is 0.62.

4. The hot rolled steel suitable for deep drawing applications in structural components as claimed in anyone of claims 1 to 3 having formability n-value varied between 0.18 and 0.22 and the r-bar varied between 0.75 and 0.79 with tensile strength in the range of 380 to 520 MPa and an elongation in the range of 22% to 30%.

5. The hot rolled steel suitable for deep drawing applications in structural components as claimed in anyone of claims 1 to 4 having good formability as qualified Erichsen cup test withno failure.

6. The hot rolled steel suitable for deep drawing applications in structural components as claimed in anyone of claims 1 to 5having elongated or fibrous single phase ferritic microstructure and wherein with higher silicon steel provide elongated grains due to ferrtic rolling in the finishing stages with formation of gamma fiber (<111>//ND) grain orientation beneficial to formability and impart favourable deep drawing characteristics.

7. The hot rolled steel suitable for deep drawing applications in structural components as claimed in anyone of claims 1 to 6 having density in the range of 7770kg/m3 (0.95%Si) and 7590kg/m3 (2.6%Si) with the steel being lighter by 1 to 3%.

8. The hot rolled steel suitable for deep drawing applications in structural components as claimed in anyone of claims 1 to 7having formability comprising plastic strain ratio (r-bar) in the range of 0.75-0.79, stretch flange ability (?%) in the range of 50-120, forming limit in the range of major strain from 0.294 to 0.607 and minor strain from -0.24 to 0.45 for a typical 0.95% Si steels and closer values for Si contents as high as 2.6%.

9. A process of manufacturing of high silicon and very low carbon hot rolled steel suitable for deep drawing automotive structural components as claimed in any one of the claims 1 to 8 comprising the steps of:
(i) providing the starting composition comprising carbonbetween 0.0005 to 0.01%, silicon between 0.95 and 2.5% and minor alloying with aluminium between 0.05 and 0.3%;
(ii) processing the starting composition for steel manufacture;
(iii) solidifying and casting the processed composition thus produced continuously into slabs;
(iv) hot rolling the slabs into hot rolled sheets by adhering to reheating temperature at 1180±50oC, roughing at temperature 1000±50oC and finishing at temperature 880+25 oC; and
(v) coiling the hot rolled sheet at temperature 650±30oC to obtain high silicon and very low carbon hot rolled steel having 1 to 6 mm thickness including single phase ferritic microstructure with high yield strength range of 250 to 350 MPa, and ultimate tensile strength (UTS) in the range of 380 to 520 MPa. suitable for deep drawing applications in structural components.
10. The process as claimed in claim 9, wherein said processing of the starting composition is selectively carried out following anyone of (i) through ladle furnace and RH degassing (ii) ladle furnace without RH degassing, where pure silicon or very low carbon ferro silicon is used as a primary alloying element and (iii) induction furnace involving commercially available pure silicon as the primary alloying element.

11. The process as claimed in anyone of claims 9 or 10, wherein the steel is made from hot metal using basic oxygen furnace or optionally electric arc furnace where low carbon is achieved and the steel is processed in secondary metallurgy route using Ladle furnace for alloying with high carbon ferro-silicon and the steel is decarburized in RH degasser for decarburization to very low carbon content.

Dated this the 7th day of October, 2020
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
HIGH SILICON AND LOW CARBON HOT ROLLED STEEL WITH EXCELLENT FORMABILITY AND MECHANICAL PROPERTIES AND A PROCESS OF MANUFACTURING THEREOF.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION

The present invention relates to hot rolled steel suitable for deep drawing applications in structural components and more specifically to very low carbon(0.0005 to 0.01%C)high silicon (0.95 to 2.5%)and Al (0.05 to 0.3%) hot rolled steel compositions and a process for their manufacturing. More particularly, the present invention is directed to a range of steel compositions comprising high silicon and aluminium along with very low carbon in hot rolled steel, adapted to ensure improved formability in the high strength range with good ductility favouring application in the production of deep drawing components used in structures of automobiles and in general engineering application and the like. Importantly, the high strength in the steel is achieved by low cost alloying elements silicon and aluminium in solid solution, which may otherwise require costly alloying elements to achieve similar strength level. The high silicon and aluminium containing hot rolled steel with a fully ferritic phase, show properties in the high strength range with improved specific strength (strength/density) and with good formability parameters of high percentage elongation, high strain hardening exponent (n), higher plastic strain ratio (r-bar), stretch flange ability (?%), and high forming limit. The steel shows good formability in spite of increased strength due to texture development associated with (111) grain orientation. These steels provide cost effective solutions for application in a variety of industries such as in the, structural components of automotive and general engineering involving the deep drawing process.

BACKGROUND OF THE INVENTION

Commercial Interstitial Free steels with very low carbon content (IF) have excellent formability, suitable for automobile application but have low strength devoid of alloying elements (strength<350MPa). Modern automotive and engineering structural applications demand, steels with high strength, lower density with very good formability is desired.The high strength steels in the present invention are alloyed with Si as a major alloying element and Al as a minor alloying element and like IF steels the carbon content is maintained at very low level similar to that of an IF steel.

The high strength steels are in general designed based on different strengthening mechanisms namely, solid solution strengthening precipitation hardening, grain size strengthening, muti-phase strengthening etc. In order to achieve the desired strengthening in the existing known art, other than carbon, various alloy elements such as chromium, nickel, titanium, manganese, vanadium, niobium, titanium, molybdenum etc, or a combination of the elements thereof are added to the steels to achieve the desired level of strengthening. Usually, with increasing strength level, the ductility and formability decreases. In the present invention a range steel of steels alloyed with major Si as alloying element and Al as additional minor alloying element and without any other alloying element and with carbon content as low as that of IF steel has been developed to exhibit high strength range. Compared to other alloying elements at equivalent high strength steels, the steels invented have cheaper alloying elements. For similar thicknesses the steel developed the steel is lighter by 1.5 to 3% in its weight due to alloying with Si and Al.

There is limited prior art on single phase ferritic high silicon containing steels for Engineering applications. Previous studies have explored evolution of metallurgical theories on solid solution hardening and mechanical behaviour rather than promoting the steel for applications involving deep drawing. The present patent explores the capability of the steels for their suitability for deep drawing and the process of manufacture adopted is commercial scale unlike laboratory investigation in most studies. The present patent covers the steel composition that has very low carbon and has also minor alloying with aluminium content. The capability of the high silicon steel for commercial deep drawing has been brought out in the invented steels developed at commercial level.

Gillet reports a high Si ferritic steel branded as Freund steel was evaluated in Germany in 1925 in a bid to promote the steel for structural application. The carbon content was around 0.12 to 0.2% along with high Si content. The steel was produced in a 3-ton open hearth furnace. The steels were evaluated in bar form by the American Bureau of standards and 0.92 to 1.92% Si containing steel with carbon around 0.12% was promoted for structural applications with yield strengths between 352 MPa and 400 MPa and tensile strengths between 493 MPa and 531 MPa with elongation 25.5 to 29% and reduction in area between 61 to 67%. The microstructure had small amounts of pearlite. These steels had significant inclusions. The present patent differs from the steel as the present steel made through RH degassing technology is much cleaner and much lower in carbon content. The present patent is developed towards formability applications that involve deep drawing. [H. W. Gillett, High silicon structural steel, technologic papers of the Bureau of Standards, No. 331, [ Part of Vol. 21], Dept of commerce, Washington, USA, 1926.]

In a classic paper on a study on alloying behaviour on mechanical behaviour of alloys, Leslie has recounted three fully ferritic steel with Si contents 0.75%, 1.5% and 3.2% (wt % Si). The base steels had composition 0.005% C-<0.01%Mn-0.002%S-0.002%P- (0.14 to 0.17) %Ti- (0.006-0.011) %Al-0.012%Ni [ wt.%]. The steels were made in a laboratory vacuum induction furnace at about 136 kg batch size and characterized. The study explored the mechanical behaviour of the steel in comparison to other binary alloyed steels. The ASTM grain size number was 6, 6.5 and between 5 and 8 respectively. He has reported that the elastic modulus, shear modulus and lattice parameter decreases by 0.5%. In binary Fe-Si alloys, highest strength is obtained with Fe-Si alloys. The yield strength is enhanced by ~10 ksi. The strength enhancement is further enhanced by higher Si contents. 3 at% Si(1.5 wt.%) gives about 20 ksi strength enhancement. Based on the atomic size misfit, highest strengthening was obtained with steels alloyed with Si content. Properties were evaluated in the steels in the steel before and after wire drawing. The 3 at% Si containing steels showed about 16 ksi tensile strength with a true strain of 6.8%. Wire drawing further enhanced the strength. This study was a laboratory scale study made towards design of the alloy which was compared with Fe alloyed with other binary alloys. The present patent deals with ferrite with Si as major alloying element and Al as minor alloying element. In addition, the steel is made in industrial production process involving a melt size of 180 Metric tonnes using Basic oxygen furnace, ladle refining and RH degassing. The processing in the present study involves continuous casting and hot rolled strips. The present study has focused on using the steel for deep drawing application. [W.C. Leslie, “Iron and Its Dilute Substitutional Solid Solutions”, The 1971 Campbell Memorial Lecture, The American Society for Metals, metallurgical transactions, volume 3, January 1972-p.5]

Davies has studied 0.98 and 1.92%Si containing fully ferritic steel at laboratory scale vacuum induction melting for understanding the mechanical behaviour of the steels. The carbon content was maintained at 0.01%. Other residual alloying elements and melt size was not reported. The study showed that the yield strength and tensile strength significantly increased with decreasing grain size. The tensile strength value of the 0.98%Si varied between 420 and 480 MPa while the 1.92%Si steel showed a range between 505 and 580 MPa. The yield strength value of the 0.98%Si varied between 270 and 350 MPa while the 1.92%Si steel showed a range between300 and 380 MPa. The true strain value of the 0.98%Si varied between 0.33 and 0.29 while the 1.92%Si steel showed a range between 0.32 and 0.26. The strength varies by 23.5 MPa per fine ness of 1 unit ASTM grain size number. This is again a laboratory study to understand mechanical behaviour of the steel. The present patent differs in manufacturing method, and delivering ferritic high Si steel with additional Al alloying and promoting the steel for deep drawing application by validation of formability parameters. [R. G. Davies, “Influence of Silicon and Phosphorous on the Mechanical Properties of Both Ferrite and Dual-Phase Steels”, Metallurgical Transactions A, Volume 10A, January, 1979, p-113]

Uenishi et al. have studied high strain rate properties of a high Si containing high strength steel sheets by alloying an IF steel with 0.98 wt.%Si and 1.97 wt.% Si. The steel was melted at 300 kg batch size in a vacuum induction melting process. The other elements in the steel was 0.0012%C-0.01%Mn-0.005%P-0.001%S-0.003%Al-0.0014 N. The strength is shown to increase with increasing strain rate in a tensile test. Increasing strain rate of deformation enhances the strength. The above was a scientific study carried out in laboratory scale while the present patent talks about industrial scale processing of a steel of commercial purity alloyed with Si as major element and Al as minor alloying elements. The present patent differs in manufacturing method, and delivering ferritic high Si steel sheet for deep drawing application. [A Uenishi, C. Teodosiu, ActaMaterialia 51 (2003),p.4437–4446].

Compared to the existing prior art, the present invention is about a range of steel compositions with very low carbon contents similar to the interstitial free or extra deep drawing type steels, but alloyed with major alloying element Si and with minor alloying element Al, which resulted in a high strength range with good specific strength and formability. The steel making process involved was ladle furnace alloying followed by RH degassing to achieve very low carbon content. The slabs were hot rolled to varying thicknesses between 1 to 6 mm.

OBJECT OF THE INVENTION

The primary object of the present invention is to provide hot rolled steel flat products, alloyed primarily with silicon as a major element and with or without aluminium as a minor alloying element, along with very low carbon content to achieve a high strength range with good formability.

The main object of the present invention is to provide hot rolled steel that exhibits high strengths with primarily silicon content and minor alloying with aluminium which impart high strength to the steel due to solid solution effect and with a very low carbon content that improves the formability of the steel.

The basic object of the present invention is directed to replace costly alloying elements usually required to develop high strength with cheaper alloying elements primarily silicon content with minor aluminium addition, which gives a completely ferritic microstructure in the high strength range with a very good formability. The steels show high yield strength, high ultimate tensile strength, low YS/UTS ratio, high strain hardening exponent (n), high plastic strain ratio (r Bar), high percentage elongation, stretch flange ability (?%), high forming limit in steel products required for deep drawing application in automotive and other sectors.

A further object of the present invention is directed to developing a low cost hot rolled steel composition wherein the addition of low cost elements primarily silicon and with or without aluminium content result in a substantial reduction in the cost of steel produced.

A still further object of the present invention is directed at providing a hot rolled steel composition comprising high silicon and aluminium addition, which favours high strength, high ductility and good formabilitydue to solid solution effect and a fine grained single phase ferritic microstructure.

A still further object of the present invention is directed at alloying predominant (111) grain oriented texture in the steel improves its formability

A still further object of the present invention is directed at alloying with high Si contents which reduces the density of the steel at high strength level, varying between 1.3 to 3.5%, which results in improved specific strength in the hot rolled steel.

A still further object of the present invention is directed at providing a hot rolled steel composition comprising high silicon content along with minor aluminium addition which forms a fully ferritic phase that is highly ductile and has superior formability at high strengths.

A still further object of the present invention is directed to providing a hot rolled steel composition comprising high silicon content which favours high hardness and low carbon content which favours lower strain aging index and good formability characteristics at high strength levels.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to hot rolled steel suitable for deep drawing applications in structural components comprising of a steel composition comprising C between 0.0005 to 0.01%, Si between 0.95 and 2.5% and minor alloying Al between 0.05 to 0.3% and rest Fe, which forms a single phase ferritic microstructure with yield strength range of 250 to 350 MPa, ultimate tensile strength (UTS) in the range of 380 to 520 MPa and elongation in the range of 23 to 30%.

A further aspect of the present invention is directed to hot rolled the steel suitable for deep drawing applications in structural components wherein the strength and specific strength of the steel increases with silicon content.
A further aspect of the present invention is directed to hot rolled steel suitable for deep drawing applications in structural components further include residual elements S<0.02%, Mn<0.6%, P<0.045%.

A still further aspect of the present invention is directed to the said hot rolled steel suitable for deep drawing applications in structural components having YS/UTS ratio is 0.62 and strain hardening exponent (n) in the range of 0.18 to 0.22.

Another aspect of the present invention is directed to the said hot rolled steel suitable for deep drawing applications in structural components having a formability n-value varying between 0.18 and 0.22 and the r-bar varying between 0.75 and 0.79 with and with tensile strength varying from 380 to 520 MPa and an elongation varying between 22% and 30%,

Yet another aspect of the present invention is directed to the said hot rolled steel suitable for deep drawing applications in structural components having good formability as qualified Erichsen cup test with no failure, stretch flangeability is in the range above 85%, the forming limit of minor strains is in the range of -0.08 to 0.23 and major strain is in the range of 0.3 to 0.48.

A further aspect of the present invention is directed to the said hot rolled steel suitable for deep drawing applications in structural components comprising of elongated or fibrous single phase ferritic microstructure and wherein with higher silicon steel provide elongated grains due to ferrtic rolling in the finishing stages with formation of gamma fiber (<111>//ND) grain orientation beneficial to formability and impart favourable deep drawing characteristics.

A still further aspect of the present invention is directed to said hot rolled steel suitable for deep drawing applications in structural components having density in the range of 7770 kg/m3(for0.95%Si) and 7590kg/m3 (for 2.6%Si) with the steel being correspondingly lighter by 1 to 3%.

A still further aspect of the present invention is directed to the said hot rolled steel suitable for deep drawing applications in structural components having formability comprising plastic strain ratio (r-bar) in the range of 0.75-0.79, percentage elongation in the range of 22 to 30, stretch flange ability (?%) in the range of 50-120, forming limit in the range of major strain from 0.294 to 0.607 and minor strain from -0.24 to 0.45 for a typical 0.95% Si steels and closer values for Si contents as high as 2.6%.

Another aspect of the present invention is directed to a process of manufacturing of high silicon and very low carbon hot rolled steel suitable for deep drawing automotive structural components as described above comprising the steps of:
(i) providing the starting composition comprising carbon between 0.0005 and 0.01%, silicon between 0.95 and 2.5% and minor alloying with aluminium between 0.05 and 0.3%;
(ii) processing the starting composition for steel manufacture ;
(iii) solidifying and casting the processed composition thus produced using continuous casting for conversion into slabs;
(iv) hot rolling the slabs into hot rolled sheets by adhering to reheating temperature at 1180±50oC, roughing at temperature 1000±50oC and finishing at temperature 880 oC ±25oC; and
(v) coiling the hot rolled sheet at temperature 650±30oC to obtain high silicon and very low carbon hot rolled steel having 1 to 6 mm thickness including single phase ferritic microstructure with high yield strength range of 250 to 350 MPa, and ultimate tensile strength (UTS) in the range of 380 to 520 MPa Elongation in the range of 22 and 30% suitable for deep drawing applications in structural components.

Another aspect of the present invention is directed to said process wherein said processing of the starting composition is selectively carried out following anyone of (i) through ladle furnace and RH degassing (ii) Ladle furnace without RH degassing, where pure silicon or very low carbon ferrosilicon is used as a primary alloying element and (iii) induction furnace involving commercially available pure silicon as the primary alloying element.

Yet another aspect of the present invention is directed to said process wherein the steel is made from hot metal using basic oxygen furnace or optionally electric arc furnace where low carbon is achieved and the steel is processed in secondary metallurgy route using Ladle furnace for alloying with high carbon ferro-silicon and the steel is decarburized in RH degasser for decarburization to very low carbon content.

A further aspect of the invention is that the strength properties in the above range of composition increases with increasing Si content without much loss in formability, while the specific strength of the steel (UTS/ density) increases due to decrease in density by 3%.

A still further aspect of the present invention is directed at the use of cheaper high carbon ferro silicon and RH degassing process to achieve the desired range of chemical composition and purity.

According to yet another advantageous aspect of the present invention, the high silicon steel which shows improved strength with good formability by enhancing carbon content from 0.005 to 0.05%C and enhanced additions of silicon which gives fully ferritic matrix that has higher strength, with ductility and formability suitable for deep drawing application.

A still further aspect of the present invention is directed to the high silicon and very low carbon hot rolled steel composition having density in the range of 7850-7600 kg/m3, that enable achievement of higher specific strength compared to conventionally alloyed steels.

Yet another aspect of the present invention, is directed to the process of manufacturing of high silicon and very low carbon hot rolled steel suitable for deep drawing applications in automotive and other structural engineering components. The process comprises the following steps. Firstly, providing the starting composition comprise of C=0.0005–0.01%, Si=0.95-2.5%, Al=0.05-0.3%, S<0.02%, Mn<0.3%, P<0.045% and rest is Fe. Secondly, processing the above composition through ladle furnace primarily alloyed with high carbon ferro-silicon followed by RH degassing. The steel may be manufactured by other routes consisting of Ladle furnace without RH degassing, where pure silicon or very low carbon ferro silicon is used as a primary alloying element. The steel may be also manufactured using induction furnace using commercially available pure silicon as the primary alloying element. Thirdly, solidifying the steel thus produced as a slab (typically 220 mm thick) through continuous casting route. Fourthly, hot rolling the slabs into hot rolled sheets, by reheating the slab for deformation, at a roughing temperature of 1180±50oC finishing temperature of 880±25oC and a coiling temperature of 880±25oC. Lastly, coiling the hot rolled sheet at a temperature 650±30oC to obtain high silicon and very low carbon hot rolled steel sheets having thickness of a range 1.8-3.0 mm, with desired properties suitable for deep drawing automotive structural components.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1: Illustrates the flow chart of a process of manufacturing of high silicon and very low carbon hot rolled steel suitable for deep drawing automotive structural components according to the present invention;

FIG. 2: Illustrates the influence of high silicon in hot rolled steel sheet on microstructure;

FIG. 3: Illustrates the influence of high silicon in hot rolled steel sheet on mechanical properties (yield strength, ultimate tensile strength, strain hardening exponent (n), percentage elongation, hardness);

FIG. 4: Illustrates the influence of high silicon in hot rolled steel sheet on formability properties at the three angular directions (stretch flange ability (?%), plastic strain ratio (r-Bar));

FIG. 5: Illustrates the influence of high silicon in hot rolled steel sheet on Erichsen Cup test;

FIG. 6: Illustrates the influence of high silicon in hot rolled steel sheet on hole expansion ratio;

FIG. 7: Illustrates the influence of high silicon in hot rolled steel sheet on forming limit;

FIG. 8: Illustrates the influence of high silicon in hot rolled steel sheet on texture;

Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The accompanying figure together with the detailed description below forms a part of the specification and serves to further illustrate various embodiments and to explain the various principles and advantages all in accordance with the present invention.

The present invention is now discussed in more detail referring to the drawings that accompany the present application. In the accompanying drawings, like and/or corresponding elements are referred to by like reference numbers.

Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or the like parts.

Before describing in detail embodiments that are in accordance with the invention, it should be observed that the embodiments reside primarily to a range of steels with high silicon contents and with minor aluminium alloying and with very low carbon content, processed as a hot rolled steel strip and a process for its manufacturing

The present invention relates to a high silicon and a very low carbon hot rolled steel composition suitable for automobile structural components, comprising C between 0.0005 and 0.01%, Si between 0.95 and 2.5% and Al between 0.05 and 0.3% made using high carbon ferrosilicon with ladle furnace + RH degassing route. The high silicon and low carbon hot rolled steel composition optionally may have residual elements S<0.02%, Mn<0.6%, P<0.045% and rest is Fe.

The high silicon and very low carbon content alloyed with minor aluminium containing steels, in the hot rolled condition is effective in providing single phase ferritic microstructure, in high strength range. Silicon is a ferrite stabilizer and it improves the strength of the steel by solid solution strengthening. The low level of carbon in the steel, suppresses the formation of carbides, which improve the formability of the steel. The high silicon and very low carbon hot rolled steel has a fully ferrite phase microstructure over the entire processing window and less austenite during initial rolling passes.

A hot-rolled high silicon steel sheet having high strength showing a tensile strength of 380 to 520 MPa and high ductility showing percentage elongation in the range of 22 to 30 can be obtained by forming a single phase structure of a ferrite phase over the entire thickness of the steel sheet. The hot-rolled thin steel sheet has excellent formability such as stretch flange ability (?%) to be formed into complicated shapes.

The high silicon and very low carbon hot rolled steel composition wherein high silicon and aluminium content is effective in providing elongated or fibrous single phase ferritic microstructure, which forms (111) texture that impart good deep drawing characteristics.

The high silicon and very low carbon hot rolled steel composition is processed to develop mechanical properties with ultimate tensile strength (UTS) in the range of 380 to 520 MPa, yield strength (YS) in the range of 250 to 350 MPa, (YS/UTS) ratio is 0.62 and strain hardening exponent (n) in the range of 0.18 to 0.22. Solid solution strengthening has been achieved in the present invention by mainly alloying with a metalloid p-orbital element Si. Thus, it is possible that atomic size variation due to Hume Rothery rule influences dislocation movement that enhances strength, and strain hardening rate (n).

The high silicon and very low carbon hot rolled steel composition have formability comprising plastic strain ratio (r-bar) in the range of 0.75-0.79, percentage elongation in the range of 22 to 30, stretch flange ability (?%) in the range of 50-120, forming limit in the range of major strain from 0.294 to 0.607and minor strain from -0.24 to 0.45 for a typical 0.95% Si steels and closer values for Si contents as high as 2.6%.

The hot-rolled steel sheet can have a tensile strength of 380 to 520 MPa and an elongation of 22% to 30%, and have desired high strength and excellent formability such as stretch flangeability (?%), formability properties, and can be formed into complicated shapes. Thus, the steel is an effective alternative to existing high strength low alloy steels.

Within the given range of Si content studied in this work, with increasing silicon content, the hardness and strength increases, while there is a marginal fall in ductility. With increasing silicon content, the strain hardening exponent and hole expansion ratio show a moderate fall while the r-bar value shows a moderate increase.

According to another incarnation of the present invention, this steel sheet or strip having a different silicon composition (%Si typically -0.95, 1.6 & 1.8) have excellent stretch flange ability(%?>50) and formability.

Alloying the steel with Si contents between 0.95 to 2.5% decreases density of the steel by 1.3 % (0.95 % Si) to 3.5 % (2.6%Si) and as a result improves the specific strength of the steel.

The process 100 of manufacturing of high silicon and very low carbon hot rolled steel suitable for automotive structural components comprising the following steps as shown in FIG.1 In step 102, the starting composition comprising carbon between 0.0005 and 0.01%; silicon content between 0.95 and 2.5% and aluminium between 0.05 to 0.3%. Optionally, the composition has residual S<0.02%, Mn<0.3%, P<0.045% and rest is Fe. Optionally, the composition can be without Al content in iron base. The steel may have residual elements Mn < 0,3% S < 0.04% and P < 0.035%.In step 104, the above composition through ladle furnace is processed. The other routes of manufacture such as lade furnace should use very low carbon ferro silicon or induction furnace should use pure silicon metal. In step 104, the composition thus produced continuously is casted into slabs of 220mm thickness. In step 106, the slabs are hot rolled into hot rolled sheets by adhering to reheating temperature at 1180±50oC, roughing at temperature 1000±50oC and finishing at temperature 880±25oC. Lastly, in step 108, hot rolled sheet is coiled at temperature 650±30°C to obtain high silicon and very low carbon hot rolled steel typically having 2.6mm thickness with desired properties suitable for deep drawing automotive structural components.

The high silicon and very low carbon hot rolled steel composition suitable for deep drawing automobile structural components is made from hot metal using basic oxygen furnace or optionally electric arc furnace where low carbon is achieved and the steel is processed in secondary metallurgy route using Ladle furnace for alloying with high carbon ferro-silicon and the steel is decarburized in RH degasser for decarburization to very low carbon content. The steel can be produced with other melting routes such as induction furnace melting, vacuum induction melting etc.1. The steel can be produced with other melting routes such as induction furnace melting, vacuum induction melting etc.

In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article or composition that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article or composition. An element proceeded by "comprises...a" does not, without more constraints, preclude the existence of additional identical elements in the process, method, article or composition that comprises the element.

In the present specification, when a part “includes” a certain component, it means that the component may further include other components, not to exclude other components, unless specifically stated otherwise.

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which the present invention pertains. Commonly used dictionary-defined terms are additionally interpreted as having meanings consistent with related technical documents and currently disclosed contents, and are not interpreted as ideal or very formal meanings unless defined.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art to which the present invention pertains can easily practice. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

In addition, unless otherwise specified, % means weight%.

Advantages and features of the present invention, and methods for achieving them will be clarified with reference to embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and only the embodiments allow the disclosure of the present invention to be complete, and are conventional in the art to which the present invention pertains. It is provided to fully inform the knowledgeable person of the scope of the invention, and the invention is only defined by the scope of the claims. The same reference numerals refer to the same components throughout the specification.

Thus, in some embodiments, well-known techniques are not specifically described to avoid obscuring the present invention. Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification may be used as meanings commonly understood by those skilled in the art to which the present invention pertains.

The present invention is described further hereinafter by reference to a series of examples.

Experiments that were actually performed are now described by way of following examples.

Evaluation 1. Manufacturing of the high silicon and very low carbon hot rolled steel

Firstly, providing the starting composition comprising C=0.0005 to 0.01%, Si=0.95 to 2.5%, Al =0.05 to 0.01% and residual elements, S<0.02%, Mn<0.3%, P<0.045% and rest is Fe. Secondly, process the above range of steel compositions, through ladle furnace alloying of commercial high carbon ferro-silicon to achieve the desired high Si levels. This is followed by RH degassing to achieve very low carbon content. Thirdly, casting the composition thus produced continuously into slabs of 220mm thickness. Fourthly, hot roll the slabs into hot rolled sheets by adhering to reheating temperature at 1180±50oC, roughing at temperature 1000±50oC and finishing at temperature 880±25oC. Lastly, coil the hot rolled sheet at temperature 650±30oC to obtain high silicon and very low carbon hot rolled steel typically having about 2.6mm thickness. Depending on applications the steel thickness can be varied. The casting conditions of the steel, is shown in Table 1 and the hot rolling condition is shown in Table 2. The composition of the steels made in the above process route is given in Table 3.

Table 1 Typical parameters of continuous casting
Conditions Values
Heat size ( tons) 180
Continuous caster type Vertical bend caster
No of strands 4
Mould dimension (mm) 220x1250 slab
Primary cooling Yes (mold cooling)
Secondary cooling Yes (stand cooling)
Mould EMS Not used
Casting speed, m/min 1.5
Super heat 25-30 °C
Mold powder brand Stollberg

Table 2 Hot rolling conditions of High Silicon steel
Conditions Values
Cast Slab dimension used for hot rolling (mm) 220 x 1250x7000 to 10000
Conditioning on the cast slab with torch Hot scarfing to 2 mm thickness
Slab Reheating temperature ( oC) 1180 + 50 oC
Slab Reheating/ soaking time ( hours) 2.5 hours
Descaling done After discharging of slab from furnace
Roughing stage
Temperature, (oC) 1000 + 50 oC
No of passes 5
Total amount of Reduction 83%
Finishing stage
Temperature, (oC) 880+ 25oC
No of stand 7
Total amount of Reduction 93%
Cooling condition at Run-out Table Automatic header on
Coiling Temperature, (oC) 650+ 30

Table 3 The typical Ladle furnace compositions of the very low carbon high silicon steel in the present invention(wt.%)
Heat C Mn S P Si Al N
1 0.0016 0.23 0.004 0.045 0.95 0.244 0.0017
2 0.0025 0.3 0.005 0.034 1.8 0.288 0.0015
3 0.0028 0.35 0.005 0.041 1.618 0.27 0.0012
4 0.0022 0.27 0.004 0.040 2.60 0.28 0.0016

Evaluation 2: Microstructure of the high silicon and very low carbon hot rolled steel

The microstructure of the steel so produced as in the manufacturing in Exam 1 was characterized for their microstructure. The hot rolled steel with different Si percentage was sampled in accordance with ASTM E3 - 11(2017) and grain size analysis in accordance to ASTM E112-2013. There were four different steel compositions studied within the range of 0.95% to 2.6% Si content. The microstructure showed a fully ferritic matrix. The surface regions show a fine equiaxed grain structure and in some cases the core has high aspect ratio elongated grain structure. The average grain size was in the range of 35 microns. The steel can be hot rolled in the austenitic region. However, in a high Si steel, when the temperature falls below 927 oC (for 0.95%Si steel) to 995oC (for 2.6%Si steel) range there is transformation of austenite to ferrite unlike other alloy steels where austenite prevails to as low as 830 oC. Hence, deformation may be performed in a temperature range of 1180+50oC. During finishing the temperature may fall below the above specified temperature ranges which imparts hot rolling in fully ferritic regime. The hot band microstructure revealed equiaxed microstructure when there is homogeneous deformation and it can exhibit elongated ferrite grains at the core as shown in FIG. 2. With increasing Si content there is a reduction in the temperature range of the austenite stability and due to this there is complete deformation in ferritic range, which leads to high anisotropic grains especially towards the steel core. The presence of this duplex grain microstructure did not seriously affect the strength and the formability characteristics of the high silicon steel as proved in the mechanical properties. FIG 2. Illustrates typical microstructure of high silicon steels showing single phase equiaxed microstructure and under some conditions the core has elongated grain structure towards the core.

Evaluation III: Mechanical propertiesof the newly developed high silicon and very low carbon hot rolled steel

Few distinct steel compositions, within the range of Si claimed in the patent have been evaluated for the mechanical properties at room temperature in the as-hot rolled strip of a chosen thickness of 2.6mm. The tensile properties were tested in accordance with ISO 6892 and the hardness in accordance with ISO 6506-2:2005.While most alloying elements were similar, four different heats with varying Si content were assessed as a function of their room temperature mechanical properties. It was observed that the steels alloyed with high Si and minor Al content exhibited high strength range with good ductility as shown in FIG. 3. The hardness, yield strength (YS0.2) and the ultimate tensile strength (UTS) increased with increasing silicon content in the steel. The Yield ratio (YS/UTS) ratio was about 0.62, which implies the steel has high ability to work harden further which eventually indicates highly formability in the steel. The ductility values show a moderate decrease with Si content. However, even at the highest Si content (2.6%), the ductility was above 20%.

As the Si content increases, the steel becomes lighter in weight. The steel becomes lighter by 1.39% for every 1%Si added. Compared to conventional alloy steels with density about 7860 kg per meter cube, the lowest Si steel in this study (0.95% Si) has a density of 7770 kg per meter cube (1.32% lighter) and the highest Si content (2.6% Si) has a density of 7590 kg per meter cube(3.56% lighter). This lowering of density enhances the specific strength (UTS/ density) of the steel. For most component designs the specific strength of the material is an important parameter. For a load bearing application, a thinner section of high Si steel can be used than equivalent strength conventional steel section. The automotive becomes lighter by usage of thinner sections of the present steels invented.

Thus, the steels in the present invention, is a light weight, high strength steel with excellent range of mechanical properties and formability characteristics.

Evaluation IV:Advanced formability characteristics of the high Si steels developed
The formability parameters n and r-bar were evaluated for the first three steels with varying Si contents as shown in FIG. 4. The n value The hot rolled steel with different Si percentage was sampled and subjected to the stretch flangeability (?%), plastic strain ratio (r-Bar) and forming limit (?) tests in accordance withISO-16630-2009, ISO 6892 and ISO 12004-2:2008 respectively. The n-value varied between 0.18 and 0.22 and the r-bar varied between 0.75 and 0.79. It is seen that the r-bar improved moderately with Si content, while the strain hardening exponent, n-value moderately decreased with Si content. It is to be appreciated that at high strength level, the r-bar has not deteriorated over the entire silicon range considered.

The steel was subjected to Erichsen cup test as shown in FIG. 5. It is observed that the steel has good formability without failure as shown in FIG.5. The steels studied show good formability. The steels were further examined for Hole expansion ratio (HER)or stretch flangebility as shown in FIG 6. It is seen that the steel has excellent hole expansion ratio, above 85% for a Si content of 1.8%. As the Si content lowered the hole expansion value increases signifying excellent formability. For automotive and related structural engineering applications involving deep drawing process a stretch flangeability of above 50% is desired. In the present range of steels invented, the stretch flangeability is above this range and as high as 120%. It is seen that the r-bar improved moderately with Si content, while the strain hardening exponent, n-value moderately decreased with Si content. It is to be appreciated that at high strength level, the r-bar has not deteriorated over the entire silicon range considered.

Forming limit diagram developed in a laboratory scale measures the ability to deep draw the steel till its failure limits is reached during sheet metal forming.The limit to which the material can be strained without breakage during deep drawing can be assessed. The forming limit diagrams, of two steels with varying Si content in FIG.7. With increase in the Si content, it is seen there is a good strain that could be accommodated in the steel before failure. Two typical steels 0.95% Si and 1.8% Si were tested for FLD. For the 1.8%Si steel, the major strain was from 0.3 to 0.57 and the minor strain was from -0.16 to 0.44. For 0.95% Si steel, the major strain was from 0.3 to 0.48 and the minor strain is from -0.08 to 0.23. This formability range is suitable for press forming operations. With increasing silicon content as strength level increases, in the steel the formability limits are not widely varying.

Evaluation VII: Evaluation of texture characteristics of the high Si steels developed

The high Si steel developed was assessed for its texture as the steels have a fully ferritic range during the final passes with significant reductions. The crystallographic texture is one of the intrinsic factor that indicates the formability of steel. The grains having gamma fiber(<111>//ND) orientation is good for formability whereas, the eta fiber (<100>//RD) oriented grains deteriorate the same. From formability behaviour, grain having (111)//ND orientations improve formability whereas, (100)//ND grains reduce it. The ratio of the gamma fiber (<111>//ND) orientated grains to eta fiber (<100>//RD) oriented grains assume importance. In a study with 0.95 %Si and 1.85% Si, the gamma fiber (<111>//ND) orientated grains are larger than the eta fiber (<100>//RD) oriented grains, which implies that the steels have better formability or deep drawing characteristics. FIG.8 illustrates the texture of the steel with predominating gama fiber which favours formability.

Evaluation VII: Bench marking the Steels developed with other commercial high strength steels

Table 4 shows the comparison of different grade of steels with high Si steel of the present invention.

Table 4: Typical composition comparable
C Mn P S Si Al V Ti Nb
Cr
Mo
440W 0.0021 0.75 0.037 0.01 0.057 0.001 0.001 0.011
HSLA -380 0.08 0.94 0.016 0.012 0.06 0.003 0.023 0.029
0.002
HSLA
ASTM A387 2 (Class 2) 0.21 0.55-0.80 0.035 0.040 0.15-0.40 0.50-0.80. 0.45-0.60
1.8%Si Steel (present invention) 0.0025 0.3 0.034 0.005 1.8 0.29 0.0002 0.0002 0.0002 0.0004

Two HSLA steel grades with about similar strength levels can be compared with a typical1.8% silicon steel developed in the present invention. The HSLA steels are alloyed by costlier Mn and microalloying elements V, Nb and Ti. All these elements are costly and have complex thermo-mechanical processing at low temperature to get favourable properties in the steel. Silicon is a much cheaper alloying element and a single phase and stronger ferrite is achieved in the steel by solid solution and with equivalent or better formability. Due to 1.8% Si alloying, the steel is lighter by 2%. A comparison of the steel with some of the commercial specifications has been made in the Table 4. It is seen that the invented steel has only cheaper Si and Al in it, compared to other elements which are low in content but they have higher cost.

Mechanical properties wise the present grade developed can go in for use where traditional high strength micro alloyed (HSLA) steels are used. It is favourable for use in the hot rolled condition for a yield strength level in the range of 330 to 450 MPa.

Table 5 Some of the specifications which can be met by the steel developed.
Steel Densitykg/m3
YS (MPa) UTS
(MPa) Specific strength %E n-value r-bar %
HER
Equivalent HSLA steel produced in JSW
440W 7870 303 507 64.42 35 0.208 1.01 75
HSLA-380 7870 440 520 66.07 26 0.15 0.7 55
Specification
SAE J2340
Gr 340Y \ 7870 340-
440 >440 55.9 24 -- -- 60
HSLA 350/450 7870 350 450 44.45 -57.18 23-27 0.14 1.1 --
A 387 2 (class 2) 7870 310 min 485-620 61.6 - 78.7 22 -- -- --
1.8%Si Steel (Present Invention) 7590 333 489 64.42 23 0.183 0.51 85

In Table 5, the steel grade 440W and HSLA -380 is being produced by JSW and it has properties equivalent to the Si steel developed in the present study. The steel also qualifies other specification in the literature. The high silicon and very low carbon hot rolled steel with 1.8% Si steel could be compared with HSLA and other grades, produced commercially. This includes 440W, HSLA-380. the steel grade with 1.8%Si meets the specifications SAE J2340, Gr 340Y, HSLA 350/450, A 387 2 (class 2).